1. Field of the Invention
The present invention relates to packet data transmission in a mobile communication system.
2. Discussion of the Related Art
Currently, high-speed downlink packet transport in a mobile communication system uses hybrid automatic repeat request (HARQ) transmission schemes, which apply channel coding to ARQ techniques, and adaptive modulation and coding (AMC), which achieves an optimal data rate by varying the modulation order and coding rate according to a current channel status. In a system adopting ARQ transmission, erroneous packets are detected at the receiving side and retransmitted according to an ACK/NACK signal fed back to the transmitting side in correspondence with each packet transfer. The feedback signal is either an acknowledgement (ACK) signal for confirming a successful instance of packet transmission or a negative acknowledgement (NACK) signal for confirming an unsuccessful instance of packet transmission. After checking the received data packets and detecting an erroneous packet, the ARQ system discards the erroneous packet, which is then wholly replaced by a retransmitted packet, but a HARQ system preserves the erroneous packet, which is combined with a correspondingly retransmitted packet, thereby achieving increased diversity gain and coding gain. While delays in the ARQ system occur when the ACK/NACK signal is transmitted by high layer signaling, the delay in the HARQ system is caused by the ACK/NACK signal being transmitted by physical layer signaling.
ARQ techniques include the stop-and-wait (SAW) method, in which a new packet is transmitted only after receiving the previous ACK/NACK signal, the go-back-N (GBN) method, in which packet transmission continues for a number of packets and a retransmission is performed for N packets preceding reception of a NACK signal, and the selective repeat (SR) method, in which only erroneous packets are retransmitted. Although implementation of the stop-and-wait method is simple, data transport efficiency suffers since each new packet must await ACK/NACK signal reception. The go-back-N method improves transport channel efficiency but is more complicated to implement. In the selective repeat method, which is the most complicated since the transmitted packets require rearrangement on the receiving side to recover their original sequencing, transport channel efficiency can be maximized.
Meanwhile, HARQ transmission schemes also retransmit a previously transmitted packet in the event of an error being present (detected) in the packet. HARQ systems, whereby a signal-to-noise ratio is increased to enable improved transport efficiency, include the chase combining (CC) method to achieve higher signal-to-noise ratios through time diversity and the incremental redundancy (IR) method to achieve higher signal-to-noise ratios through coding diversity. Chase combining employs multiple channels to transmit the packets, such that a channel for retransmission packets in the event of packet error detection differs from the channel used for previously transported packets. Each retransmission of a packet using incremental redundancy, on the other hand, applies a different (incremented) redundancy. Thus, incremental redundancy is characterized in that one packet is transported with various versions, such that if transmission of a packet of a first version fails, the packet is transmitted as a second or third version. For example, for a code rate of 1/3, a transmitted packet x can be sent as three versions, namely, x1, x2, or x3, but for a code rate of 1/2, the transmitted packets include versions x1 and x2. Assuming that, at a code rate of 1/2, a transmission of versions x1 and x2 each fails, a transmitter can send another two versions, namely, x2 and x3. Hence, from the viewpoint of the receiving side, the 1/2 code rate is changed to a code rate of 1/3.
HARQ is applicable for packet transmission in an uplink, i.e., a user entity transmitting to a base station. When the user entity communicates with multiple base stations, as in the case of a soft handover, each of the base stations determines the presence or absence of an error in the transmitted packet and accordingly transmits an ACK/NACK signal in a downlink. Thus, if each of the base stations transmits an ACK/NACK signal to the user entity, the user entity detects multiple ACK/NACK signals coming from the various base stations. Under softer handover conditions, however, where the user entity moves between the sectors of a single cell, one base station receives multiple transmissions of the same packet, which is transmitted from the user entity to each of the sectors and is then separately transferred from each sector to the base station. Hence, if transmission status at the receiving side, i.e., the state of an ACK/NACK signal to be transmitted by a downlink, is determined by decoding the packets transmitted via the respective sectors and in turn each of the sectors transmits the ACK/NACK signal to the user entity, the efficiency of ACK/NACK signal transmission is lowered.